The proposed experiments are designed to continue an investigation of mechanisms of learning in the mammalian brain. It has previously been shown that both the hippocampal pyramidal cells and entate granule cells exhibit learning-dependent changes in neuronal activity during nictitting membrane (NM) conditioning in rabbit. While conditioned activity emerges in both the granule and pyramidal cells within the first eight trials, the conditioned responses in dentate differ in amplitude, pattern, and latency from those in the CA3 area of hipposampus. These results bring into question the role of the very strong dentate to CA3 projection. Here the PI proposes to investigate further the conditioned responses in dentate by recording granule cell activity during acquisition and reversal of two-tone discriminative NM conditioning. Extracellular single unit recordings will be made from dentate granule cells during the acquisition (Experiment 1) and reversal (Experiment 2) phases of the conditioning paradigm. Units will be characterized by their spike width and waveform, spontzneous firing rate, and response to electrical stimulation of the perforant path. Results from this study will form the basis for future studies of the contribution of dentate granule cells to the conditioned responses in CA3, the behavioral significance of this contribution, and the role of structures afferent to dentate in the development of the conditioned dentate response. For this series of experiments the PI wishes to utilize a behavioral paradigm which is sensitive to manipulations of the hippocampus. While hippocampal elsions do not disrupt simple NM conditioning, Berger and Orr have recently shown that performance on reversal of two-tone NM conditioning is severely disrupted in animals with hippocampal lesions. Therefore the PI proposes to use the reversal paradigm to continue his investigations into the conditioned activity of hippocampus and its primary inputs. The results should increase our knowledge of the function of the limbic system in learning and memory processes.